1. Selected Measurements
Particle/Surface Interactions Investigated with TIRM
G. Brügger, L. Schaffner, J. Ríčka, and M.Frenz
Dept. of Biomedical Photonics, University of Bern;
Evanescent Wave Illumination/TIRM
total internal reflection (TIRM) when angle of incidence qi higher than critical angle qc → “evanescent wave” is generated
Intensity of evanescent wave gets exponentially weaker with increasing distance to surface and particles scatter light proportional to this intensity
→ evanescent wave illuminates only particles very close to the surface
→ possible to determine particle/surface separation by particle brightness
Optical Tweezers
particles with refractive index higher than surrounding medium can be trapped with a gaussian laser beam
depending on beam shape, particles will be trapped around focus or pushed away
Possible Measurements
observation of ad- and desorption of single particles to surface → deposition statistics, particle counting [1]
total fluorescence signal → determination of surface mass density of fluorescence marked molecules [2]
interaction between single particle (trapped by optical tweezers) and surface, intensity of scattered light contains information about weak repulsive and attractive forces → interaction potential can be restored [3]
Selected Measurements
particle adsorption on different surfaces
glass Supported Phospholipid Bilayer (SPB) encourages particle adsorption
protein adsorption onto glass
adsorbed amount can be determined absolutely from subsequent flushing at the end of the experiment
single particle pushed by optical trap against glass
fluctuations due to Brownian motion allow determination of interaction potential
References
L. Schaffner et al., “Surfactant mediated adsorption of negatively charged latex particles to a cellulose surface”, Colloids and Surfaces A: Physicochem. Eng. Aspects 286 (2006), 39–50
V. Hlady, D. R. Reinecke, J. D. Andrade, “Fluorescence of Adsorbed Protein Layers”, Journal of Colloid and Interface Science 111 (1986),
Dennis C. Prieve, “Measurement of colloidal forces with TIRM”, Advances in Colloid and Interface Science 82 (1999),